This invention relates to synthetic auxetic materials, that is, to polymeric materials having a negative Poisson ratio whereby, when stretched in one direction by application of a tensile load, the material expands transversely to that direction. Alternatively, when compressed in one direction, the material contracts transversely to that direction.
Synthetic auxetic materials-have been known since 1987. In the first instance, and as described in U.S. Pat. No. 4,668,557, auxetic materials were prepared as open-celled polymeric foam, negative Poisson ratio properties being obtained as a consequence of mechanical deformation of the foam by compression.
More recently, auxetic materials have been formed as polymer gels, carbon fibre composite laminates, metallic foams, honeycombs and microporous polymers.
Published patent specification WO 91/01210 describes a polymeric material having an auxetic microstructure of fibrils interconnected at nodes. As described, this material is obtained by a process which comprises compacting polymer particles at elevated temperatures and pressures and then deforming the compacted polymer by draw-assisted extrusion through a die to produce a cylindrical rod of auxetic material. A typical process may use a compaction stage with a specially designed processing rig heated to 110-125° C. with a blank die inserted. Polymer powder is added into a barrel of the rig and is allowed to come to temperature for between 3-10 minutes before compaction pressure is applied with a ram at a rate of up to 140 mm/min. The pressure is held at 0.04 GPa for 10-20 minutes and the resulting rod of compacted material is then removed from the barrel of the processing rig and allowed to air cool. The processing rig is then fitted with an extrusion die in place of the blank die and heated up to 160° C. The compacted rod is reinserted into the barrel and sintered at 160° C. for 20 minutes. It is then immediately extruded at a rate of 500 mm/min at 160° C. through a conical die of geometry entry diameter 15 mm, exit diameter 7-7.5 mm, cone semi-angle 30° and capillary length 3.4 mm.
The material obtained from this typical process has auxetic properties derived from the microstructure of the material, namely fibrils interconnected at nodes capable of transverse expansion and increased porosity when the material is stretched.
Auxetic materials are of interest as a consequence of predicted enhancement of mechanical properties such as plane strain fracture toughness and shear modulus. This enhancement has been demonstrated in practice in tests in terms of indentation resistance and ultrasound attenuation with blocks of auxetic microporous ultra high molecular weight polyethylene.
Enhancements in hardness of up to three times at low loads, and very large enhancements (again up to three times) in the attenuation coefficient (i.e. how much of an ultrasound signal is absorbed) are exhibited as between the auxetic material and conventional polyethylene.
Known auxetic materials have been made in the form of bodies with relatively low aspect ratios, and in the case of auxetic microporous polymers these have been made from powder using a three stage process involving compaction, sintering and ram extrusion through a conical die as described above.
Hitherto therefore auxetic materials have not successfully been made in the form of fibres (i.e. with high aspect ratios), despite the interest in such materials. Typically a fibre is an elongate body having a length at least 100 times its diameter.
An object of the present invention is to provide a viable process for the production of auxetic materials in fibre form.
According to one aspect of the invention therefore there is provided an auxetic polymeric material which is of filamentary or fibrous form.
According to a further aspect of the invention therefore there is provided a method of forming an auxetic material comprising cohering and extruding heated thermoformable particulate polymeric material, wherein cohesion and extrusion is effected with spinning to produce filamentary material having auxetic properties.
With this arrangement, surprisingly the use of spinning with cohesion and extrusion provides an effective means of producing auxetic material in filamentary form. It has been found that this process can provide an auxetic microstructure of fibrils and nodes, as with the above mentioned prior art process, but without requiring the separate compaction and sintering stages of the prior art.